The human visual system has achieved the theoretical physical limit of sensitivity to light: absorption of a single quantum of light is an event that can be detected by the cells in the retina. This remarkable sensitivity is due in part to properties of the light-sensitive cells themselves, the photoreceptors, and in part to the ways in which other retinal cells process the electrical signals generated by the photoreceptors. The overall aim of the proposal research is to gather quantitative information about the generation and analysis of these electrical signals in the vertebrate retina. The first step on vision is the translation of light energy into an electrical signal that can be passed along to other neurons in the retina. Understanding of this process of visual transduction is fundamental to an understanding of human vision, and a number of proposed experiments are aimed at the mechanism of transduction. In most experiments, the focus is on inferences about the transduction process drawn from analysis of the electrical responses to light in individuals photoreceptors. These responses will be monitored by recording changes in the electrical current flowing across the membrane of the photoreceptor, a technique that is suffciently sensitive to detect the absorption of a single quantum of light. Orther experiments will test the degree to which the scheme for photoreceptor function emerging from studies of cold-blooded vertebrates can be extended to mammalian retina. Another line of research will investigate the ways in which the responses of photoreceptors are modified as they are passed on through the retina for eventual relay to the brain.